The present invention relates generally to a brake system master cylinder in a hydraulic brake system for an automotive vehicle, and in particular to a quick take-up master cylinder for disc-brake system. More specifically, the invention relates to a blow-off valve assembly in a quick take-up master cylinder.
In automotive vehicle brake systems, disc-brake systems are becoming widely used, since they do not tend to over-heat and cause vapour lock in the brake hydraulic circuit. In order to prevent the brake hydraulic circuit from over-heating and from generating noise, it is necessary to provide a sufficient space between the brake pad and a brake disc mounted and secured on the wheel axle for rotation therewith. In such brake systems, a quick take-up master cylinder is generally utilized to improve the response characteristics of the braking operation.
A typical construction of a quick take-up master cylinder is disclosed in the U.S. Pat. No. 4,133,178, issued on Jan. 9, 1979 to Frank W. Brooks, Sr. In this reference, a quick take-up master cylinder is disclosed which has a low pressure, high volume displacement, quick take-up chamber, and a high pressure, low volume displacement pressurizing chamber formed by a stepped bore and a stepped piston. A compensation control and blow-off valve unit is disclosed having a peripheral lip seal type valve providing compensation flow on brake release but preventing flow from the quick take-up chamber when the brakes are applied. A normally closed check valve is held open while the master cylinder is in the fully released position to provide communication between both pressurizing chambers and the fluid reservoir. As the pressurizing cup for the high pressure chamber closes its compensation port, the check valve is permitted to close by a camming action between a valve pin and a camming surface on the piston. Initial flow is obtained from the quick take-up chamber past the pressurizing cup into the high pressure chamber. At a predetermined pressure level, arranged to be sufficient to take up lining clearance and initially apply the vehicle brakes, the pressure in the quick take-up chamber opens the normally closed check valve so that fluid from the quick take-up chamber then flows to the reservoir without further pressure buildup in that chamber. As the master cylinder is released, compensation fluid can flow past the peripheral compensation valve to both chambers. When the master cylinder is returned to the fully released position, the check valve is forced open by action of the piston cam surface and the valve pin to provide fluid communication between the reservoir and both pressurizing chambers. A fluid flow restrictive orifice is provided in series with the normally closed check valve to prevent the high rate of pressure increase in the quick take-up chamber from being bypassed fully to the fluid reservoir upon a high rate of master cylinder actuation, forcing at least some of the pressure to pass by the cup seal into the high pressure chamber and the brake apply circuit.
FIGS. 1 illustrates the relationship between input force applied to the brake pedal and an output force applied to the wheel cylinders and FIG. 2 illustrates the relatioinship between the stroke of the primary and secondary pistons and fluid amount supplied to each of the wheel cylinders. As apparent from FIGS. 1 and 2, at the initial stage of application of the brake force, the amount of fluid supplied to the wheel cylinder rapidly increases while the brake pressure applied thereto moderately increases. Specifically, by rapidly reducing the volume of the primary fluid chamber, the amount of fluid to be supplied to rear wheel cylinder is rapidly increased. In spite of increasing the amount of fluid supplied to the rear wheel cylinder, the pressure of the fluid is not so rapidly as a result of communication between the primary pressure chamber and the fluid reservoir via a compensation passage. The brake positions are represented by lines a and b in FIGS. 1 and 2 respectively. When the primary piston blocks communication between the primary pressure chamber and the fluid reservoir, the fluid pressure in the primary pressure chamber increases rapidly as represented by line c in FIG. 1. Corresponding to the increasing of the fluid pressure in the primary pressure chamber, the secondary piston is moved to rapidly increase the fluid pressure therein. Therefore, as seen from FIG. 1, a loss stroke of brake operation indicated at L remains even for quick take-up master cylinders.